1. Field of Invention
The present invention relates to a power amplifier, and more particularly to a high efficiency power amplifier.
2. Description of Related Art
As is well known, an amplifier is a device that receives an input and applies a defined gain in order to produce an output that is greater than the input. For example, a voltage amplifier may receive a 10V input and produce an output of 20V, if the voltage amplifier has a gain of two. Amplifiers are able to apply the gain to produce an output up to a certain value. For example, a voltage amplifier can produce an output up to a value of the operational voltage applied to the amplifier. That is, a voltage amplifier with an operational voltage of 15V and a gain of three can produce the desired output as long as the input does not exceed 5V. If the voltage amplifier receives an input voltage of 6V it attempts to produce an output of 18V, which is above the maximum that this voltage amplifier can produce. Therefore, the voltage amplifier does not have the potential to drive the load to the desired voltage.
Amplifiers work best if they produce outputs near the maximum output of the amplifier. That is, an amplifier with an operational voltage of 15V works most efficiently when producing outputs around 15V. However, the typical outputs of a 15V amplifier are probably much lower since the 15V operating voltage (i.e., maximum output) is selected to handle peak input voltages. For example, the maximum input voltage (peak voltage) for a 15V amplifier with a gain of three is 5V, and a typical input voltage may be in the range of 1-3V.
FIG. 1 illustrates a schematic diagram of a conventional sine wave amplifier system. The power circuit 101 transfers the received AC power or DC power to a required operation voltage supplied to the amplifier circuit 102. The amplifier circuit 102 receives the sine wave signal supplied from the small signal producer 103 and produces an output according the defined gain of the amplifier circuit 102. FIG. 2 is a schematic diagram illustrating a conventional Class B amplifier circuit 102. Other amplifier circuits, such as Class A, Class AB or other types of amplifier circuits, may use the same method described in the following for analysis. First, this amplifier circuit 102 is powered by a pair of operation voltages (+V and xe2x88x92V) supplied from the power circuit 101. The transistor Q1 is responsible for amplifying the received signal during period T1, and the transistor Q2 is responsible for amplifying the received signal during period T2. Both Q1 and Q2 transistors operate in a linear region.
FIG. 3 is a waveform diagram further illustrating the operation of the conventional sine wave amplifier system. The amplifier circuit 102 receives a small signal supplied from the small signal producer 103 and produces a sine wave output according to the defined gain. The amplifier circuit 102 is powered by a pair of operation voltage sources (+V and xe2x88x92V) supplied from the power circuit 101. As the chart depicts, the typical maximum output voltage Vp is required to be between +V and xe2x88x92V. The sine wave output signal VR is represented by the following equation:
VR=VPxc3x97sin(2xcfx80ft)
Therefore, the output efficiency varies with the output signal waveform. Typically, the amplifier circuit 102 runs at peak efficiency periodically when peak output voltage Vp appears. Therefore, the sine wave amplifier system as shown in FIG. 1 is very inefficient. On the other hand, the voltage difference (Vce) between the collector electrode and the emitter electrode of the transistors Q1 and Q2 is changed when the output signal changes. The power dissipation is as follows:
powerdissipation=Vcexc3x97Icxe2x80x83xe2x80x83(1)
Vce is the voltage difference between the collector electrode and the emitter electrode of the transistor. Ic is the current of the collector electrode. The equations shown in the following describe the voltage Vce and the current Ic of the transistors Q1 and Q2, respectively:                     V        ce            ⁢              (                  Q          1                )              =          V      -                        V          p                xc3x97                  sin          ⁢                      (                          2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              f              ⁢                              xe2x80x83                            ⁢              t                        )                                                  V        ce            ⁢              (                  Q          2                )              =          V      +                        V          p                xc3x97        sin        ⁢                  (                      2            ⁢                          xe2x80x83                        ⁢            π            ⁢                          xe2x80x83                        ⁢            f            ⁢                          xe2x80x83                        ⁢            t                    )                                                                            I              c                        ⁡                          (                              Q                1                            )                                =                      xe2x80x83                    ⁢                                    V              p                        xc3x97                                          sin                ⁢                                  (                                      2                    ⁢                                          xe2x80x83                                        ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    f                    ⁢                                          xe2x80x83                                        ⁢                    t                                    )                                            /              R                                                                        xe2x80x83                    ⁢                                    sin              ⁢                              (                                  2                  ⁢                                      xe2x80x83                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                  f                  ⁢                                      xe2x80x83                                    ⁢                  t                                )                                       greater than             0                                                                                  I              c                        ⁡                          (                              Q                1                            )                                =                      xe2x80x83                    ⁢          0                                                  xe2x80x83                    ⁢                                    sin              ⁢                              (                                  2                  ⁢                                      xe2x80x83                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                  f                  ⁢                                      xe2x80x83                                    ⁢                  t                                )                                       less than             0                                                                                  I              c                        ⁡                          (                              Q                2                            )                                =                      xe2x80x83                    ⁢                                    V              p                        xc3x97                                          sin                ⁢                                  (                                      2                    ⁢                                          xe2x80x83                                        ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    f                    ⁢                                          xe2x80x83                                        ⁢                    t                                    )                                            /              R                                                                        xe2x80x83                    ⁢                                    sin              ⁢                              (                                  2                  ⁢                                      xe2x80x83                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                  f                  ⁢                                      xe2x80x83                                    ⁢                  t                                )                                       less than             0                                                                                  I              c                        ⁡                          (                              Q                2                            )                                =                      xe2x80x83                    ⁢          0                                                  xe2x80x83                    ⁢                                    sin              ⁢                              (                                  2                  ⁢                                      xe2x80x83                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                  f                  ⁢                                      xe2x80x83                                    ⁢                  t                                )                                       greater than             0                              
In accordance with equation (1), the power dissipation of the transistors is as follows:
powerdissipation=Vce(Q1)xc3x97Ic(Q1)+Vce(Q2)xc3x97Ic(Q2)
In accordance with the above equation, the power dissipation is about 30% to 70%.
Thus, an inherent problem associated with standard amplifiers is the conflict between the desirability of providing large output potentials and the undesirability of providing lower potentials through a large potential drop. One solution is provided in FIG. 4. FIG. 4 illustrates a schematic diagram of an amplifier system in accordance with the conventional analog to digital amplifier system. The power circuit 401 transfers the received AC power or DC power to the required operation voltage supplied to the analog to digital amplifier circuit 402. The analog to digital amplifier circuit 402 receives the sine wave signal supplied from the small signal producer 403 and produces a pulse-width-modulation (PWM) wave, in which the analog to digital amplifier circuit 402 is controlled by a PWM signal that is provided by the PWM signal producer 404. Then, the PWM wave passes through the semiconductor switch 405 and is provided to the wave filter, which comprises an inductor L and a capacitor C, to produce an output wave that is enlarged and in the same phase with the input sine wave signal. Although such an analog to digital amplifier system may obtain a high output efficiency, the wave filter requires a high-value inductor and thus occupies a large area and raises the power dissipation.
According to the above descriptions, because the conventional amplifier system needs to handle the peak outputs, the output efficiency cannot attain an optimal state. Even though the output efficiency may be raised by such an analog to digital amplifier system, this analog to digital amplifier system requires a wave filter that is composed of an inductor L and a capacitor C to produce an enlarged output wave in the same phase with the input sine wave signal. Although such an analog to digital amplifier system may obtain high output efficiency, the wave filter requires a high-value inductor and thus occupies a large area and raises the power dissipation. Therefore, this present invention provides a new amplifier system structure to overcome the above drawbacks, such as low output efficiency and large area occupation.
Typically, the efficiency of an amplifier circuit can be improved by dynamically changing the level of the supply voltages. That is, the level of the supply voltages is changed in response to changes in the level of the input analog signal. The goal of this strategy is to minimize the voltages supplied to the amplifier circuit to avoid extra power dissipation. In other words, the level of the supply voltages is changed in response to changes in the level of the input analog signal. Therefore, when a peak signal must be transmitted, the supply voltages supply at their respective high levels. When a lower level signal must be transmitted, the supply voltages supply at their respective lower levels. Dynamically changing the level of the supply voltages minimizes the power dissipation.
In accordance with the above description, the main purpose of the present invention is to provide an amplifier system that dynamically changes the level of the supply voltages.
Another purpose of the present invention is to provide an amplifier system for receiving an input analog signal and generating a corresponding output analog signal.
A further purpose of the present invention is to provide an amplifier system that dynamically changes the level of the supply voltages to enlarge any kind of signal with a minimum power dissipation.
Yet another purpose of the present invention is to provide an amplifier system that can handle a peak input voltage while maintaining high output efficiency at any time.
To accomplish this, an amplifier system of the present invention comprises a rectifier circuit, two switching circuits, a D.C.-to-A.C. circuit, a sine wave producer and logic control circuit, a pulse-width-modulation and phase signal producer circuit, a feedback circuit and a linear amplifier circuit. The rectifier circuit, two switching circuits and a D.C.-to-A.C. circuit comprise the power circuit of the amplifier system in accordance with the present invention.
The pulse-width-modulation and phase signal producer circuit generate a series of pulse-width-modulation (PWM) signals. These PWM signals are used to control the switching circuits and the D.C.-to-A.C. circuit, respectively, to transform the received AC or DC power into dynamic power supplied to the amplifier circuit. The amplifier circuit receives the input sine wave generated by the sine wave producer and logic control circuit and generates a corresponding output sine wave that is greater than the input. The feedback circuit connects the amplifier circuit and the pulse-width-modulation and phase signal producer circuit. The feedback circuit transmits the dynamic power supplied to the amplifier circuit to the pulse-width-modulation and phase signal producer circuit to modulate the PWM signal. The modulated PWM signal ensures that the dynamic power keeps the same phase as the input sine wave and follows the voltage curve of the input sine wave.
On the other hand, because the present invention may produce a dynamic power according to the input signal, the efficiency of the amplifier circuit can be significantly improved by dynamically changing the level of the supply voltages.